In recent years, a swallow-type capsule endoscope has appeared in the field of endoscope. The capsule endoscope has an imaging function and a radio communication function. After a patient swallows the capsule endoscope from the mouth for observation (examination), the capsule endoscope moves within the body cavity such as internal organs, for example a stomach and a small intestine following peristaltic motions thereof, and sequentially images, until the capsule endoscope is naturally discharged from the human body.
During a period while the capsule endoscope moves within the body cavity, image data picked up by the capsule endoscope inside the body is sequentially transmitted to the outside by radio communication, and is stored in a memory. The patient carries a receiver having the radio communication function and the memory function. With this arrangement, after the patient swallows the capsule endoscope, the patient can move freely, until the capsule endoscope is discharged. After the capsule endoscope is discharged, a doctor or a nurse can perform diagnosis by displaying the images of the organ on a display, based on the image data stored in the memory.
While the capsule endoscope can have such a configuration that driving power is supplied from an incorporated power source, a configuration which calls attention recently supplies driving power to the capsule endoscope from the outside via radio transmission. Based on the configuration in which the power is supplied from the outside, it is possible to avoid such a situation in which the driving of the capsule endoscope stops in the middle of the move in the body cavity due to an unintended exhaustion of power (see, for example, Patent Document 1).
FIG. 16 is a circuit diagram showing a configuration of a receiving antenna incorporated in the conventional capsule endoscope to receive radio-transmitted power. As shown in FIG. 16, the receiving antenna includes a receiving resonance circuit 203 having a receiving coil 201 and a receiving capacitor 202, a rectifying diode 204 that converts an alternate-current signal into a direct-current signal, and a storage capacitor 205 that stores power rectified by the rectifying diode 204. The receiving resonance circuit 203 includes the receiving coil 201 and the receiving capacitor 202 so as to have a resonance frequency that matches the frequency of a transmitted electric signal. The rectifying diode 204 rectifies the received electric signal, and the storage capacitor 205 stores the rectified electric signal. Each constituent element of the capsule endoscope operates using the power stored in the storage capacitor 205 as driving power.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-231186 (Page 3, and FIG. 1)